This paper reports the results of the utilization of a silica waste from a geothermal power generation plant as replacement material of Portland cement. Pastes of OPC substituted with 0, 10, 15 and 20% of geothermal waste and water/solids ratio of 0.50 were cured at 20 and 40C for up to six months. The pastes were characterised by thermogravimetry, compressive strength, non-evaporable water, and scanning electron microscopy. The results show that the presence of the silica enhances the compressive strength, which was also favoured by higher temperatures in the short term; however, at silica loads above 15% an anomalous strength reduction was noted; fracture microstructures under scanning electron microscopy showed the presence of Aft needle clusters. The geothermal silica shows pozzolanic character according to CH measurements and it also influenced the cement hydration in agreement with non-evaporable water results. The microstructures were more compact in the presence of geothermal silica.
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